Can't connect to backend server if they are using SSL with wildcard Subject Alternative Names

You almost certainly need to send SNI to some of those servers, so they actually respond.

You do this by adding sni str(example.org) :

backend app5
 http-request set-header Host txt2html.sourceforge.net
 server server5 txt2html.sourceforge.net:443 ssl sni str(txt2html.sourceforge.net) verify none